As an industrial process for producing aminocarboxylic acid salts, a Strecker process has been generally employed in which glycine salts, iminodiacetic acid salts or nitrilotriacetic acid salts are obtained using hydrocyanic acid and formaldehyde as starting materials. Since, however, hydrocyanic acid is a deadly poisonous gas, the process is greatly limited in production equipment, handling, location area, etc. Moreover, because hydrocyanic acid is mostly obtained as a byproduct in producing acrylonitrile, there has been a big problem in the aspect of securing safety of starting materials.
Also known is a process in which aminocarboxylic acid salts are produced by oxidatively dehydrogenating amino alcohols in alkali hydroxides (U.S. Pat. Nos. 2,384,816, 2,384,817, 3,535,373, 3,842,081 and 3,739,021, etc.) U.S. Pat. No. 2,384,816 discloses a process in which an amino alcohol is reacted with an alkali metal hydroxide in the absence of a catalyst. In this process, however, a reaction time is long and a yield of the aminocarboxylic acid salt is low. U.S. Pat. No. 2,384,817 discloses a process in which potassium glycinate is formed by anhydrously reacting monoethanolamine with potassium hydroxide in the presence of a copper catalyst. In this process, however, according to the present inventors' knowledge, a yield of the glycinate is not good. U.S. Pat. No. 3,578,709 discloses a process in which a nitrilotriacetic acid salt is formed by reacting triethanolamine with an alkali hydroxide in the presence of a zinc oxide catalyst. This process is, nevertheless, unsatisfactory in a yield of the nitrilotriacetic acid salt. U.S. Pat. No. 3,842,081 discloses that potassium iminodiacetate is obtained in a relatively high yield by reacting diethanolamine with potassium hydroxide in the presence of cadmium oxide. U.S. Pat. Nos. 3,535,373, 3,578,709 and 3,739,021 disclose that a nitrilotriacetic acid salt is formed by reacting triethanolamine with an alkali hydroxide in the presence of cadmium oxide. However, because of a risk of incorporating a poisonous cadmium compound into a reaction product, these processes using cadmium oxide as a catalyst cannot be used at all depending on the use and suffer a problem of waste matters; they cannot become a technology competitive with the Strecker process.
A process is further known in which an amino-carboxylic acid salt is obtained by reacting an amino alcohol in the presence of an alkali hydroxide, water and a copper-containing catalyst or a catalyst containing copper and zirconium (U.S. Pat. No. 4,782,183). In the process, selectivity to the aminocarboxylic acid salt is indeed as high as 95 %, but the repeated use of the catalyst tends to decrease selectivity and increase by-products. Main by-products are an oxalic acid salt when producing a glycine salt using monoethanolamine as a starting material, a glycine salt when producing an iminodiacetic acid salt using diethanolamine as a starting material, and an iminodiacetic acid salt and a glycine salt when producing a nitrilotriacetic acid salt using triethanolamine as a starting material. Accordingly, in order to obtain the aminocarboxylic acid with good selectivity, there is a need to exchange the catalyst for a short period of time or to recover the catalyst by a complex purification step.
It is an object of this invention to provide a novel process, which is free from a toxicological problem, which gives little by-products, which is high in yield and selectivity, which allows the repeated use of the catalyst, and which can therefore produce an aminocarboxylic acid salt economically advantageously.